Though today there exist a vast variety of solutions for digital data storage, there still is no reliable and accepted solution for long term archival of digital data. Nevertheless there clearly are needs to store digital data for long periods of time, e.g. digital movie data, but also medical data or contractual or client-related data of companies.
One possible solution for long term archival consists in storing the digital data on a two-dimensional surface, e.g. on film or paper. Typically the data are recorded in the form of two-dimensional data patterns consisting of an array of black and white pixels. However, also grey value or color coded pixel arrays are used.
More generally, the data are stored on a two-dimensional surface with pattern elements forming a regular pattern. A specific value is assigned to each element during an encoding process, e.g. a color, a grey value etc. If one pattern element shall store one bit, one of only two possible values is assigned to the pattern element, e.g. the values black and white. The easiest way of decoding the bit stored in a pattern element is to look at the element and to check its value, e.g. if the element is black or white. Unfortunately, due to interference with neighboring pattern elements, e.g. caused by the low pass characteristic of the reading channel, blurring, inter symbol interference, or other distortions like aging, the values of the retrieved pattern elements are often different from the originally stored pattern elements. For example, upon retrieval of the pattern elements the initial values “black” and “white” become “grey”. As a result it sometimes is rather difficult to reliably determine the initial values of the pattern elements.